The present invention relates in general to contact printing and, more particularly, to a method of coating a mask in a photolithographic process.
The fabrication of integrated circuits involves the creation of several layers of materials that interact in some fashion. One or more of these layers may be patterned so various regions of the layer have different electrical characteristics, which may be interconnected within the layer or to other layers to create electrical components and circuits. These regions may be created by selectively introducing or removing various materials. The patterns that define such regions may be created by lithographic processes. For example, a layer of photoresist material is applied onto a layer overlying a wafer substrate. A mask (containing clear and opaque areas) is used to selectively expose this photoresist material by a form of radiation, such as ultraviolet light, electrons, or x-rays. Either the photoresist material exposed to the light, or that not exposed to the light, is removed by the application of a solvent. An etch may then be applied to the layer not protected by the remaining resist, and when the resist is removed, the layer overlying the substrate is patterned.
In this photolithographic process, the mask may be placed in contact with the photoresist (contact printing), may be placed close to the photoresist (proximity printing), or may be projected over a larger distance (projection printing). The contact printing method was probably the earliest method used to produce patterns on integrated circuits and may provide excellent resolution and high throughput. The mask must be correctly aligned on the photoresist/layer, e.g., when previous patterns have been created. The mask is secured to the photoresist covered layer by clamping or vacuum, for example.
However, for contact printing, defects in the pattern or the mask are routinely experienced, especially when the mask is used repeatedly to print several substrates consecutively without cleaning the mask. These defects print on the next layer that is exposed to the mask. Consequently, hard surface masks used in contact printing must be inspected and cleaned regularly, a time consuming and expensive process. And if the defects cannot be eliminated, the masks must be replaced.
These defects in contact printing occur when particles of the photoresist, being tacky or sticky in nature, stick to the mask. Conventional masks comprise a chrome and glass surface, with the radiation passing through the glass but not the chrome. Glass and chrome both have a high surface energy and readily induce particles of photoresist and dirt to cling thereto. When particles stick to the glass, the radiation is absorbed (blocked) and does not reach the photoresist on subsequent layers being processed. Failure of the radiation to reach these small areas on subsequent layers of photoresist creates defects. Furthermore, particles sticking to either the glass or the chrome prevent close contact (creating a gap) between the mask and the photoresist surface, reducing resolution of the resulting image. As resolution is reduced, the lines of demarcation between areas become less defined. Therefore, it is desired to have minimal diffraction induced resolution loss.
Efforts in the past to resolve these problems include applying a thin, one molecule deep, conversion coating (fluorinated hydrocarbon chlorosilane monomers) to the mask. This coating would bond to the glass portion of the mask, but has less affinity to bond with the chrome portion of the mask. Test results using a small number of samples showed that this coating had very limited durability. As a result of this lack of durability, the coating would have to be reapplied after only a few uses.
Therefore, an improved method of contact printing is desired that greatly reduces defects by reducing the number of particles attaching to the mask during the photolithography process.